[avr-gcc-list] int (*256) optimization problem in avr-gcc 4.5.3

[Paweł Stawicki]

Hi all,

I found a issue in the optimization part of gcc-avr.

(I'm new to this stuff so be patient :) )

I'm losing some data returned from a function,

Attaching the smallest example, I could find.

Similarly bug occurs in the ArduinoISP example,

function program_page().

code:

unsigned char getch();

int f()

{

    int a;

    unsigned char c2;

    a = getch();

    a *= 256;  //!! bug - getch() value disappears

    c2 = getch();

    a  += c2;

    return a;

}

output: 

int f()

{

   0: ef 92       push r14

   2: ff 92       push r15

   4: cf 93       push r28

   6: df 93       push r29

    int a;

    unsigned char c2;

    a = getch();

   8: 0e 94 00 00 call 0 ; 0x0 <_Z1fv>

    a *= 256;  //!! bug - getch() value disappears

   c: 80 e0       ldi r24, 0x00 ; 0

   e: ec 01       movw r28, r24

    c2 = getch();

  10: 0e 94 00 00 call 0 ; 0x0 <_Z1fv>

    a  += c2;

  14: c8 0f       add r28, r24

  16: d1 1d       adc r29, r1

    return a;

}

  18: ce 01       movw r24, r28

  1a: df 91       pop r29

  1c: cf 91       pop r28

  1e: ff 90       pop r15

  20: ef 90       pop r14

  22: 08 95       ret

some info:

system ubuntu 11.10 amd64

#avr-g++ -mmcu=atmega1280 -g -Os -c b.cpp

#avr-objdump -S  -d b.o > b.asm

#avr-g++ -v

Using built-in specs.

COLLECT_GCC=avr-g++

COLLECT_LTO_WRAPPER=/usr/lib/gcc/avr/4.5.3/lto-wrapper

Target: avr

Configured with: ../src/configure -v --enable-languages=c,c++ --prefix=/usr/lib --infodir=/usr/share/info --mandir=/usr/share/man --bindir=/usr/bin --libexecdir=/usr/lib --libdir=/usr/lib --enable-shared --with-system-zlib --enable-long-long --enable-nls --without-included-gettext --disable-checking --disable-libssp --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=avr

Thread model: single

gcc version 4.5.3 (GCC) 

Best regards

Paweł,